Bandpass filter circuit

ABSTRACT

A bandpass filter circuit includes first and second resistors connected in series between input and output terminals thereof, an amplifier having a gain of three and its output connected to the output terminal, and an RC circuit network connected between the connection point of the first and second resistors and the input of the amplifier and having bandpass characteristics. The RC circuit network comprises a series combination of a third resistor and a first capacitor connected between the connection point of the first and second resistors and the input of the amplifier, and a parallel combination of a fourth resistor and a second capacitor connected between the input of the amplifier and a common terminal of the filter circuit. The third and fourth resistors have an equal resistance value and the first and second capacitors have an equal capacitance value. As one of the first and second resistors use may be made of a variable resistor to change the quality factor, and the third and fourth resistors may be variable resistors ganged with each other to change the center frequency of the passband.

BACKGROUND OF THE INVENTION

The present invention relates to a filter circuit and, more particularly, to an active bandpass filter circuit capable of controlling independently the quality factor (Q) and center frequency of the filter circuit.

A graphic equalizer or a tone controller generally employed in audio systems includes a bandpass filter circuit for controlling the response of a speaker or listening room according to music being played. The filter circuit is usually arranged to independently adjust the center frequency and the quality factor thereof. It is preferable that such filter circuit can easily and independently adjust the center frequency and quality factor (Q value) thereof, receive music signals, whose dynamic range or allowable amplitude range is wide, without generating distortion, and have a simplified circuit configuration.

A prior art bandpass filter circuit shown in FIG. 1 comprises a band elimination filter circuit 10 and a subtraction circuit 30. The band elimination filter circuit 10 includes a twin-T network 11 having variable resistors 12-14 and capacitors 15-17 configured as shown in FIG. 1, first and second buffer amplifiers 18, 19 each having a gain of unity, and a potentiometer 20. The subtraction circuit 30, which serves to subtractively combine input and output signals of band elimination filter circuit 10 to provide bandpass characteristic, includes an operational amplifier 31 and resistors 32 to 35. Variable resistors 12, 13 and 14 are ganged with one another and adapted to adjust the center frequency of a pass band of filter circuit. The potentiometer 20 is adapted to adjust the quality factor (Q).

For the purpose of obtaining desired frequency responses by this prior art bandpass filter circuit, resistors 12 to 14 and capacitors 15 to 17 must be under a severe relation to have such values, respectively, as shown in FIG. 1. It is also very difficult to adjust values of variable resistors 12, 13 and 14 in an accurately associated manner to change the center frequency. Gang error is caused because of mismatching of variable resistors 12, 13 and 14, thereby deteriorating the bandpass characteristic. Gang error increases exponentially with the increased number of elements to be ganged.

FIG. 2 shows another prior art band elimination filter circuit 10 which may be used as the band elimination filter of the bandpass filter circuit shown in FIG. 1 which includes an operational amplifier 21, a buffer amplifier 22 having a gain of unity, resistors 23, 24, a potentiometer 25, a series circuit 36 of variable resistor 37 and capacitor 38, and a parallel circuit 39 of variable resistor 40 and capacitor 41. Variable resistors 37 and 40 are ganged with each other and adapted to adjust the center frequency of the bandpass filter. The potentiometer 25 is adapted to adjust the quality factor.

In the case of filter shown in FIG. 2, only two variable resistors may be ganged with each other to adjust the center frequency, so that gang error is reduced to some extent as compared with the circuit of FIG. 1. However, the circuit of FIG. 2 has such a drawback that the dynamic range of input signals is limited lower than half the maximum output voltage of amplifier 21 because the gain of operational amplifier 21 (two in this case) is determined by the ratio of resistors 23 and 24.

Both the bandpass filter circuit of FIG. 1 and the one in which the band elimination circuit of FIG. 2 is employed are complicated in circuit configuration to thereby increase cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a filter circuit having a simple circuit configuration and capable of controlling the center frequency and quality factor easily and independently.

Another object of the present invention is to provide a filter circuit having a simple circuit configuration and capable of receiving input signals whose allowable amplitude range is comparatively wide.

A filter circuit embodying the present invention includes first and second resistors connected in series between input and output terminals of filter circuit, an amplifier having a gain of three having its output connected to the input terminal of the filter circuit, and an RC circuit network connected between the connection point of the first and second resistors and the input of the amplifier, said RC circuit network comprising a series combination of a third resistor and a capacitor connected between the connection point of first and second resistors and the input of the amplifier, and a parallel combination of a fourth resistor and a second capacitor connected between the input of the amplifier and a common terminal of the filter circuit.

The third and fourth resistors have a substantially equal resistance value and the first and second capacitors have a substantially equal capacitance value.

The ratio of resistance values of the first and second resistors may be changed to vary the quality factor of the filter circuit. One of the first and second resistors may therefore be of variable type.

Third and fourth resistors or first and second capacitors may be changed together to change the center frequency of the passband.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a prior art bandpass filter circuit.

FIG. 2 shows another example of band elimination filter shown in FIG. 1.

FIG. 3 is a schematic circuit diagram showing a filter circuit embodying the present invention.

FIG. 4 shows in detail the block 56 of filter circuit shown in FIG. 3.

FIG. 5 shows a practical arrangement of the filter circuit shown in FIG. 3.

FIG. 6 shows the frequency responses of a filter circuit of the present invention.

FIG. 7 is a schematic circuit diagram of a frequency characteristic adjusting circuit in which the filter circuit of the present invention may be employed.

FIGS. 8, 9 and 10 show the frequency response for explaining the frequency characteristic adjusting circuit shown in FIG. 7.

FIG. 11 shows another example of a frequency characteristic adjusting circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 3, a bandpass filter 50 embodying the present invention includes a variable resistor 53 having a resistance value of Rb and a resistor 54 of a resistance value Ra, both resistors being connected in series between an input terminal 51 and an output terminal 52; a filter circuit 56 having its input terminal 56a connected to a connection point between resistors 53 and 54, and its common terminal 56c connected to ground; and an amplifier 57 having a gain of three and having its input terminal connected to an output terminal 56b of filter 56 and its output terminal connected to the output terminal 52.

The filter circuit 56 employed in the present invention comprises, as shown in FIG. 4, a series circuit 58 of a variable resistor 59 and a capacitor 60 connected between the input and output terminals 56a and 56b, and a parallel circuit 61 of a variable resistor 62 and a capacitor 63 connected between the output and common terminals 56b and 56c. Variable resistors 59 and 62 are ganged with each other as shown in FIG. 4.

Providing that each value of variable resistors 59 and 62 be represented by R and that each value of capacitors 60 and 63 by C, the transfer function T(jω) of filter circuit of FIG. 4 will be expressed by the ratio between the output and input voltages V₂ and V₁ as follows: ##EQU1## where ω₀ =1/CR and f₀ =ω₀ /2π. This transfer function shows a bandpass characteristic in which the peak value of the output voltage V₂ is a third of the input voltage V₁ at the resonance frequency f₀. The resonance frequency f₀ can be easily changed by adjusting the variable resistors 59 and 62. Alternatively, capacitors 60 and 63 may be adjusted in the ganged manner to change the resonance frequency f₀.

Providing that an input voltage at the input terminal 51 is Vi and that an output voltage at the output terminal 52 is V₀ in the operation of filter circuit 50 of FIG. 3, the following equation is given

    (Vi-V.sub.1)/Rb=(V.sub.1 -V.sub.0)/Ra                      (2)

An input current of the filter circuit 50 is neglected in this case.

The output voltage V₂ of the filter circuit 56 is amplified by a factor of 3 by means of the amplifier 57. Therefore,

    V.sub.2 =V.sub.0 /3                                        (3)

From equations (1), (2) and (3) ##EQU2## With G⁻¹ =Vi/V₀, equation (4) can be rewritten as follows: ##EQU3## Further with k=(Ra+Rb)/Ra, equation (5) can be rewritten as follows: ##EQU4## Substituting equation (1) into equation (6) yields ##EQU5## Since the transfer function G(jω) of the filter circuit 50 is G(jω)=V₀ /Vi=1/G⁻¹, ##EQU6## where k/3 represents the quality factor (Q value) of the bandpass filter. As apparent from the foregoing, center frequency f₀ of the bandpass filter circuit 50 can be changed by changing the CR time constant of the filter 56 while the quality factor (Q value) can be changed by varying the ratio of the variable resistor 53 and the resistor 54 in the bandpass filter circuit of the invention.

The dynamic range of the input signal will be now described. When the input signal frequency is equal to the center frequency f₀, the gain of the filter circuit is unity, that is, the output voltage V₀ of the amplifier 57 is equal to the input voltage Vi thereof. Therefore, the allowable input voltage is equal to the maximum output voltage determined by the power supply voltage of amplifier 57. When the input signal frequency is different from the center frequency f₀, the gain of the filter circuit becomes smaller than unity, so that the allowable input voltage of the filter circuit 50 determined by the maximum output voltage of the amplifier 57 becomes larger than the maximum output voltage of the amplifier 57. The larger the value of the variable resistor 53 or the Q value is, the larger the allowable input voltage becomes in this case.

FIG. 5 shows a preferred embodiment of the bandpass filter circuit of FIG. 3. The same parts as those in FIGS. 3 and 4 are represented by the same reference numerals. This embodiment includes an operational amplifier 64 connected between the connection point of the variable resistor 53 and the resistor 54 and the input terminal 56a of filter 56. The amplifier 57 uses an operational amplifier 65 having its noninverting input connected to the output terminal 56b of filter 56, its inverting input connected via a resistor 66 (resistance value r) to the common terminal 56c of the filter 56, and its output connected to the output terminal 52 of the filter circuit 50 and via a resistor 67 (resistance value 2r) to the inverting input thereof. Addition of the amplifier 64, which has a high input impedance, enables a bandpass characteristic close to the theoretical characteristic to be obtained.

FIG. 6 shows transfer characteristics obtained by various Q values when f₀ =1 kHz, R=3.2 KΩ, and C=0.05 μF.

FIG. 7 shows a frequency response adjusting circuit in which the filter circuit of the present invention is employed and which comprises an inverting amplifier 70 comprised of an operational amplifier 71, an input resistor 72 (resistance value R₁) and a feedback resistor 73 (resistance value R₁) for amplifying an input voltage at an input terminal 74; an inverting amplifier 75 comprised of an operational amplifier 76, an input resistor 77 (resistance value R₁) and a feedback resistor 78 (resistance value R₁) for amplifying an output signal of the inverting amplifier 70; a variable resistor 80 having fixed terminals 80a and 80b connected to the inverting inputs of operational amplifiers 71 and 76 and its center tap grounded; and the filter circuit 50 of this invention having the input terminal 51 connected to the output of operational amplifier 71, the output terminal 52 connected to a slider terminal 80c of the variable resistor, and the common terminal 56c grounded. Frequency responses shown in FIGS. 8 to 10 are obtained at an output terminal 79 of this frequency response adjusting circuit.

Namely, the quality factor variable characteristic shown in FIG. 8 is provided by changing the variable resistor 53 of filter 50 and the center frequency variable characteristic shown in FIG. 9 is provided by changing both variable resistors 59 and 62. Dip amount is changed, as shown in FIG. 10, by shifting the slider terminal 80c of the variable resistor 80 from the center tap thereof to the fixed terminal 80a, while peak amount is varied by shifting the slider terminal 80c from the center tap to the fixed terminal 80b.

FIG. 11 shows another example of a frequency characteristic adjusting circuit in which the filter circuit 50 of this invention is employed and which comprises an operational amplifier 81 having its non-inverting input connected via a resistor 82 (resistance value R₁) to an input terminal 83, its output connected to an output terminal 84 and its inverting input grounded; an operational amplifier 85 having its inverting input connected via an input resistor 86 (resistance value R₁) to the output terminal 84, its output connected via a feedback resistor 87 (resistance value R₁) to the inverting input thereof and via a resistor 88 (resistance value R₁) to the non-inverting input of operational amplifier 81 and its non-inverting input grounded; a variable resistor 90 having fixed terminals 90a, 90b connected to the non-inverting input and inverting input of operational amplifiers 81, 85, respectively, and whose center tap is grounded; and the filter circuit 50 having the input terminal 51 connected to the output of operational amplifier 85, the output terminal 52 connected to a slider terminal 90c of variable resistor 90, and the common terminal 56c grounded.

Filter characteristics shown in FIGS. 8 to 10 are also provided by this frequency characteristic adjusting circuit.

The resistor 54 may be of variable type to change quality factor instead of resistor 53 in the bandpass filter shown in FIG. 3. Capacitors 60, 63 may be of variable type to change center frequency instead of resistors 59, 62 in the filter shown in FIG. 4. 

What is claimed is:
 1. A filter circuit having input and output terminals and a common terminal, comprising:a first combination circuit having first and second resistors connected in series between said input and output terminals; a second combination circuit having a third resistor and a first capacitor connected in series between two terminals, said second combination circuit being connected to the connection point of said first and second resistors at one of said two terminals; a third combination circuit having a fourth resistor and a second capacitor connected in parallel between the common terminal of said filter circuit and the other terminal of said second combination circuit; and a non-inverting amplifier having input and output and a gain of three, said input of said amplifier being connected to an output of said second and third combination circuits, and said output of said amplifier being connected to said output terminal of said filter circuit; wherein an output signal of said second and third combination circuits is applied to an input side of said amplifier to be non-inverting amplified with a gain of three, an output signal of said amplifier which has a same phase with and a magnitude three-times as great as that of the output signal of said second and third combination circuits is applied to said output terminal of said filter circuit, and a divided output voltage which is obtained by dividing a voltage between an input signal voltage which is applied to said input terminal of said filter circuit and the output signal voltage which is derived at the output terminal of said filter circuit and has the same phase as that of the output signal of said second and third combination circuits is applied to an input side of said second and third combination circuits; and wherein the quality factor of said filter circuit is determined by a ratio of a resistance value of said first resistor to a resistance value of said second resistor, and the gain of said filter circuit is always unity at a resonance frequency of said second and third combination circuits. PG,16
 2. A filter circuit according to claim 1 wherein one of said first and second resistors is a variable type resistor.
 3. A filter circuit according to claim 1 wherein said third and fourth resistors have resistance values substantially equal to each other and said first and second capacitors have capacitance values substantially equal to each other.
 4. A filter circuit according to claim 1 or 3 wherein said third and fourth resistors are ganged with each other. 